Astronomers have spotted two supermassive black holes
in the crowded center of a distant galaxy. And it's only a matter
of time, they say, before the pair collide.

Nov. 20, 2002:
For the first time, scientists have found proof of two supermassive
black holes together in the same galaxy. These black holes are
orbiting each other and will merge several hundred million years
from now. The event will unleash intense radiation and gravitational
waves ... and leave behind an even larger black hole than before.

NASA's Chandra X-ray Observatory spotted the two black holes
in the galaxy NGC 6240. The observatory was able to "see"
them because the black holes are surrounded by hot swirling vortices
of matter called accretion disks. Such disks are strong sources
of x-rays.

Above: An artist's concept of two black holes (surrounded
by glowing hot accretion disks) about to merge. [see
the animation]

"The breakthrough came with Chandra's ability to
clearly distinguish the two nuclei and to measure the details
of their X-radiation," said Guenther Hasinger of the Max
Planck Institute for Extraterrestrial Physics in Germany. He's
a coauthor of an upcoming paper in the Astrophysical Journal
Letters describing the research.

At a distance of about 400 million light-years, NGC 6240 is
a prime example of a starburst galaxy--that is, a massive galaxy
in which stars are forming at an exceptionally rapid rate due
to a recent collision and subsequent merger of two smaller galaxies.
Because of the large amount of dust and gas in such galaxies,
it is difficult to peer deep into their central regions. Optical,
radio, and infrared observations had previously revealed two
bright nuclei, but their nature was a mystery.

X-rays, however, can penetrate the veil of gas and dust. "With
Chandra, we hoped to determine which one, if either, of the nuclei
was an active supermassive black hole," said Stefanie Komossa,
also of the Max Planck Institute, lead author of the paper on
NGC 6240. "Much to our surprise, we found that both were
active black holes!"

Left:
A composite optical (Hubble) and x-ray (Chandra) photo of the
inner regions of galaxy NGC 6240. X-rays from the two supermassive
black holes appear blue in this false-color image. [more]

"The detection of a binary black hole supports the idea
that black holes can grow to enormous masses in the centers of
galaxies by merging with other black holes," said Komossa.
"This is important for understanding how galaxies form and
evolve," she said.

Over the course of the next few hundred million years, the
two black holes in NGC 6240, which are about 3000 light-years
apart, will drift toward one another and merge to form an even
larger supermassive black hole. Toward the end of this process
an enormous burst of gravitational waves will be produced.

These gravitational waves will spread through the universe
and produce ripples in the fabric of space, which would appear
as minute changes in the distance between any two points. NASA's
planned space-based detector, LISA (Laser
Interferometer Space Antenna), will search for gravitational
waves from massive black-hole mergers. These events are estimated
to occur several times each year in the observable universe.

"This is the first time we see a binary black hole in
action, the smoking gun for something that will become a major
gravitational wave burst in the future," said Hasinger.

Credits: Chandra observed NGC 6240 for 10.3 hours with
the Advanced CCD Imaging Spectrometer (ACIS). Other members of
the team are Vadim Burwitz and Peter Predehl of the Max Planck
Institute, Jelle Kaastra of the Space Research Organization Netherlands
and Yasushi Ikebe of the University of Maryland in Baltimore.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages
the Chandra program for the Office of Space Science, Washington,
and TRW, Inc., Redondo Beach, Calif., is the prime contractor
for the spacecraft. The Smithsonian's Chandra X-ray Center controls
science and flight operations from Cambridge, Mass.

A Mystery in the Galactic Center -- (Science@NASA) Astronomers have learned
that the center of our Milky Way galaxy harbors a long-sought
black hole. But the finding has raised even more questions than
before.